Glass optical fibers have very small diameters and are susceptible to external influences such as mechanical stress and environmental conditions. To protect the fiber from such influences, it is provided with one or more coatings of a of protective material.
Certain uses of optical fibers require that a portion of the coating be removed from an end of the fiber or a portion of the fiber that is remote from the ends. For example, to make a fiber optic coupler, coating is stripped from portions of at least two optical fibers, and the stripped portions are fused together in side-by-side relationship and stretched. In one type of coupler referred to as an overclad coupler, the stripped portions of the fibers are inserted into a tube which is collapsed onto the fibers and stretched. Regardless of how the coupler is made, it is inserted into a protective housing from which fiber pigtails extend for the purpose of connecting the coupler to other optical fibers. It is important that the stripped portions of the fibers do not become weakened during the stripping process since weakened fibers can fail during subsequent process steps or during handling of the coupler when tensile stress is applied to the pigtails.
The process of fusion splicing together two optical fibers or two multi-fiber cables requires that the ends of each pair of fibers that are to be joined be stripped before they are fusion spliced. The spliced portions of the fibers are then recoated. It is increasingly necessary to make splices of reliably high strength.
Therefore, the coating stripping process should not inordinately reduce fiber strength.
Coating can be manually stripped from an optical fiber by placing the fiber within a hand-held tool, bringing blades of the tool into contact with opposite sides of the coating layer, and then moving the tool relative to the axis of the coated optical fiber. The bare portion of the fiber usually needs to be wiped with a cloth wetted with alcohol or the like to remove smudges and/or particles of coating that have been deposited on the bare portion of optical fiber by the coating removal process. This type of coating removal process has been built into equipment that performs the tasks of the technician, whereby the process is no longer manual.
Some coating removal processes employ a solvent such as acetone to soften and swell the coating so that it is more easily removed. Alternatively, a heater within the coating removal apparatus can also cause softening and swelling of the coating. Coating can also be removed by contacting the coated fiber with materials such as sulfuric acid and hydrogen peroxide (to remove a polyester coating) or with sulfuric acid (to dissolve a polysyloxine coating).
These processes have various disadvantages. Processes that are adapted to remove coating only from the end of a coated fiber are often not useful for removing coating from regions remote from the fiber ends, a requirement for making fiber optic couplers. Mechanical strippers can cause scoring or scratching of the fiber surface. For example, the fiber surface can be scratched as the bare glass fiber without its coating is removed from the mechanical stripping device. Some stripping methods do not result in a clean demarcation between the stripped coating and the coating remaining on the fiber. If the stripping process results in a ragged or uneven coating termination, the resultant sripped fiber may not be useful for its intended purpose. Mechanical stripping can also contaminate the fiber surface, thus requiring a subsequent cleaning step to remove particulates. The wiping step can cause more damage to the fiber than the mechanical stripping step. Mechanical stripping that is manually performed is highly operator dependent as it is typically performed with a hand held stripper.
Chemical stripping processes can be disadvantages in that they are difficult to control, and they may leave contaminants or residual coating on the fiber surface. Chemical stripping can also be disadvantages in that it utilizes dangerous caustaic chemicals that need to be handled and disposed of. Moreover, chemicals can flow between the fiber and the coating that remains on the fiber just beyond the stripped region.
It is particularly difficult to remove the coating from an array of optical fibers such as a ribbon cable or a circularly symmetric multi-fiber cable since coating material is situated between closely spaced fibers or between fibers and filamentary strength members in such cables.